For many years it has been known that the worn interior surfaces of aircraft cylinder assemblies can be rebuilt by plating those surfaces with a layer of chromium in a special way which produces a micro-cracked pattern on the chromium so that the chromium layer will be wetted by oil. See for instance the following United States patents:
Patent No. Inventor ______________________________________ 1,441,468 C. H. Wills 2,048,578 H. Van Der Horst 2,412,698 H. Van Der Horst 2,433,457 T. C. Jarrett, et al. 2,856,344 S. D. Lapham 2,980,593 C. R. Larson 3,192,618 G. A. Altgelt ______________________________________
The aircraft cylinder assembly generally comprises a thin walled steel cylinder which receives the piston of a reciprocating piston engine with the outer end of the steel cylinder attached to an aluminum head portion which carries suitable valve seats, fuel inlet and exhaust passageways and the like. In the commercial application of this known method of rebuilding, it has been conventional to dip the head portion of the cylinder assembly in wax to a sufficient depth that the interior surfaces of the aluminum head are coated with wax before plating. The wax layer on the aluminum surfaces prevents plating of the chromium on the head portion, thereby limiting the chromium layer to the desired interior area of the cylinder. The "waxed" cylinder assembly is attached to an anode and the combination is then lowered into a chromium plating bath and plated for sufficient time to build up the desired chromium thickness of the interior of the cylinder. Thereafter the combination is removed from the plating bath and post treated to develop the micro-cracks. The cylinder assembly is then removed from the anode and the wax is removed to provide a clean part for subsequent honing, grit blasting and the like.
The necessity for the waxing operation in this method of rebuilding cylinder assemblies has created serious problems. Thus, the manual steps involved in applying and removing the wax coating are time consuming and expensive. More importantly, the waxing operation has been a source of errors which are responsible for the rejection of rebuilt cylinders. For instance, a microscopic particle splashed in the interior surface of the cylinder during the waxing operation will produce a pit in the chromium layer, and incomplete removal of the wax prior to grit blasting can cause the entrapment of grit particles in screw threads with resulting damage to the rebuilt assembly when screws are inserted in the threads. There is also the possibility that entrapped grit will be released into the engine's lubrication system during engine operation.